Method of purifying alkaline-earth and alkali-earth halides for crystal growth

Abstract

An improved technique that reduces the potential for trapped impurities and/or ensuring desired stoichiometry of a grown crystal. Improved contaminant removal is obtained by bubbling a scavenger gas, such as fluorine gas or hydrogen fluoride gas, through a melt of alkaline- or alkali-earth halides, to improve the purity of the melt by removing more volatile metal halides and oxygen contained within the melt. By reacting after the raw material has melted, any oxygen or metal impurities trapped in the raw material is free to react with the scavenger. A desired stoichiometry is achieved as the alkaline- or alkali-earth metals react with the halide in the scavenger gas. Decreasing the amount of impurities in the melt, and using a desired stoichiometeric melt, improves the radiation hardness and transmission properties of resulting ingot grown from the purified raw material. Additionally, this method may decrease the amount of time needed for outgassing. The method may also be used to form a high purity pre-melt, which in turn may be used to grow an ingot with higher purity.

1. A method of treating a growth material used to form halide crystals, comprising the steps of:
 heating a growth material comprising an alkaline- or alkali-earth halide to form a melt; and 
 bubbling a scavenger gas through the melt. 
 
   
   
     2. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of sodium in the growth material to less than 2.5 parts per million. 
   
   
     3. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of sodium in the growth material to less than about 2.0 parts per million. 
   
   
     4. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of sodium in the growth material to less than about 1.8 parts per million. 
   
   
     5. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of silicon in the growth material to less than 1.4 parts per million. 
   
   
     6. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of silicon in the growth material to less than about 1.0 parts per million. 
   
   
     7. A method as set forth in  claim 1 , wherein the scavenger gas is bubbled through the melt for a period of time sufficient to reduce the presence of silicon in the growth material to less than about 0.7 parts per million. 
   
   
     8. A method as set forth in  claim 1 , wherein the growth material prior to being melted contains at least one impurity, and scavenger gas is bubbled through the melt for a period of time sufficient to reduce the concentration of the impurity in the growth material by at least about 25%. 
   
   
     9. A method as set forth in  claim 8 , wherein the impurity is at least one of sodium, silicon, aluminum, iron and zinc. 
   
   
     10. A method as set forth in  claim 1 , wherein the growth material prior to being melted contains at least one impurity, and scavenger gas is bubbled through the melt for a period of time sufficient to reduce the concentration of the impurity in the growth material by at least about 50%. 
   
   
     11. A method as set forth in  claim 10 , wherein the impurity is at least one of sodium, silicon, aluminum, iron and zinc. 
   
   
     12. A method as set forth in  claim 1 , wherein the growth material is selected from a group consisting of calcium fluoride, barium fluoride, magnesium fluoride, strontium fluoride, sodium fluoride, cesium fluoride and lithium fluoride, calcium chloride, barium chloride, magnesium chloride, strontium chloride, sodium chloride, lithium chloride, cesium chloride, calcium bromide, barium bromide, magnesium bromide, strontium bromide, sodium bromide, lithium bromide, cesium bromide, calcium iodide, barium iodide, magnesium iodide, strontium iodide, sodium iodide, lithium iodide, cesium iodide, and mixtures thereof. 
   
   
     13. A method of growing a halide crystal, comprising the step of growing a halide crystal from the purified growth material obtained in accordance with the method of  claim 12 . 
   
   
     14. A method as set forth in  claim 1 , wherein the growth material is loaded into and heated in a crucible, the crucible is heated within an enclosed space, and reaction gases produced by reaction of the scavenger gas with impurities in the growth material are exhausted from the enclosed space. 
   
   
     15. A method as set forth in  claim 1 , further comprising the step of cooling the melt to form a pre-growth material that can be subsequently reheated to form a melt from which a crystal can be grown. 
   
   
     16. A method as set forth in  claim 15 , further comprising the step of loading the pre-growth material in a crucible, heating the crucible to melt the pre-growth ingot, and growing a crystal from the melt. 
   
   
     17. A method as set forth in  claim 1 , wherein the heating step includes heating the alkaline- or alkali-earth halide in an enclosed space, and further comprising the step of exhausting from the enclosed space reaction gases produced by reaction of the scavenger gas with impurities in the growth material. 
   
   
     18. A method as set forth in  claim 1 , wherein the scavenger gas is selected from a group consisting of fluorine gas, hydrogen fluoride gas, nitrogen trifluoride, carbon tetrafluoride, sulfur hexafluoride, trifluroomethane, iodine gas, hydrogen iodide, tetraiodomethane, triiodomethane, bromine gas, hydrogen bromide, tetrabromomethane, chlorine gas, hydrogen chloride gas, tetrachloromethane gas, trichloromethane gas, and mixtures thereof. 
   
   
     19. A method as set forth in  claim 1 , wherein the growth material is selected from a group consisting of calcium fluoride, barium fluoride, magnesium fluoride, strontium fluoride, sodium fluoride, cesium fluoride and lithium fluoride, and mixtures thereof. 
   
   
     20. A method of growing a fluoride crystal, comprising the step of growing a fluoride crystal from the purified growth material obtained in accordance with the method of  claim 19 .